In the semiconductor art it is often desirable to form dielectric layers or films on various supporting structures, such as the gate insulator in field effect transistors, an insulator or passivation layer covering various areas (e.g. the extrinsic base region) of other types of transistors, such as HBTs and the like, an insulator or passivation layer surrounding the mesa or walls of a vertical cavity surface emitting laser or edge emitting lasers, etc. Regardless of the use, it is generally imperative that the dielectric layer or film be a good insulator with low defect density to enable device operation and enhance/maintain device performance. Also, the thickness of the layer must be sufficient to provide the required characteristics of the semiconductor devices, e.g. leakage current, reliability, etc.
Due to a lack of insulating layers having low interface state density and stable device operation on gallium arsenide (GaAs) based semiconductors, the performance, integration level and marketability of both digital and analog GaAs based devices and circuits is significantly limited. As is known in the art, growing oxide films by oxidizing GaAs based materials results in high interface state density and a Fermi level which is pinned at the GaAs-oxide interface.
A method of forming a thin film of Ga2O3 is disclosed, for example, in M. Passlack et al., Journal of Vacuum Science & Technology, vol. 17, 49 (1999), and U.S. Pat. Nos. 6,030,453 and 6,094,295. As discussed in these references, a high quality Ga2O3/GaAs interface is fabricated using in situ deposition of gallium oxide molecules on GaAs based epitaxial layers while maintaining an ultra-high vacuum (UHV). The thus fabricated Ga2O3—GaAs interfaces have interface recombination velocities S of 5,000-30,000 cm/s and interface state densities Dit as low as 3.5×1010 cm−2 eV−1. However, the properties of gallium oxides fabricated by this technique are inadequate for many applications because of high oxide bulk trap densities and excessive leakage current. Consequently, the performance of unipolar and bipolar devices is affected and the fabrication of stable and reliable metal-oxide-semiconductor field effect transistors (MOSFET) based on compound semiconductors has been problematic.
As discussed in U.S. Pat. No. 6,159,834, it has been determined that the aforementioned technique does not produce a high quality Ga2O3 layer because of oxygen vacancies in the layer that give rise to defects that cause unacceptable oxide trap densities. The '834 patent overcomes this problem by directing a molecular beam of gallium oxide onto the surface of the wafer structure to initiate the oxide deposition, and a second beam of atomic oxygen is supplied upon completion of the first 1-2 monolayers of Ga2O3. The molecular beam of gallium oxide is provided by thermal evaporation from a crystalline Ga2O3 or gallate source, and the atomic beam of oxygen is provided by any one of RF or microwave plasma discharge, thermal dissociation, or a neutral electron stimulated desorption atom source. This fabrication technique increases the quality of the Ga2O3 layer by reducing the density of oxygen related oxide defects while maintaining the excellent quality of the Ga2O3—GaAs interface. However, oxide bulk trap densities are still unacceptably high and significant leakage current is observed.
As an alternative to Ga2O3, gadolinium gallium oxides (Ga2O3(Gd2O3)) have been employed as a dielectric layer on GaAs-based devices. While this oxide layer has an acceptably low leakage current density, Ga2O3(Gd2O3)—GaAs interface state densities are relatively high, resulting in unacceptable device performance.
Accordingly, it would be desirable to provide a dielectric layer structure on GaAs-based devices that has both a low defect density oxide-GaAs interface and a low oxide leakage current density.